Knowledge of Zend like libraries are probably recommended to set you apart from other applicants, but it isn't required. The library you will need to know is based on the job you are applying for.

My personal philosophy is that understanding how to code without the libraries, and having a strong OOPHP knowledge is better than knowing a specific library. Add to that a strong knowledge of the client server relationship of PHP and JavaScript (If you are trying to call PHP functions in your JS or PHP in your JS you fail), and you will be golden.